Re: [PATCH v4] mm/memory_hotplug: Fix remove_memory() lockdep splat

[David Hildenbrand <david@redhat.com>]

On 11.01.20 15:25, David Hildenbrand wrote:
> 
> 
>> Am 11.01.2020 um 14:56 schrieb Qian Cai <cai@lca.pw>:
>>
>> 
>>
>>> On Jan 11, 2020, at 6:03 AM, David Hildenbrand <david@redhat.com> wrote:
>>>
>>> So I just remember why I think this (and the previously reported done
>>> for ACPI DIMMs) are false positives. The actual locking order is
>>>
>>> onlining/offlining from user space:
>>>
>>> kn->count -> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock
>>>
>>> memory removal:
>>>
>>> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock -> kn->count
>>>
>>>
>>> This looks like a locking inversion - but it's not. Whenever we come via
>>> user space we do a mutex_trylock(), which resolves this issue by backing
>>> up. The device_hotplug_lock will prevent
>>>
>>> I have no clue why the device_hotplug_lock does not pop up in the
>>> lockdep report here. Sounds wrong to me.
>>>
>>> I think this is a false positive and not stable material.
>>
>> The point is that there are other paths does kn->count —> cpu_hotplug_lock without needing device_hotplug_lock to race with memory removal.
>>
>> kmem_cache_shrink_all+0x50/0x100 (cpu_hotplug_lock.rw_sem/mem_hotplug_lock.rw_sem)
>> shrink_store+0x34/0x60
>> slab_attr_store+0x6c/0x170
>> sysfs_kf_write+0x70/0xb0
>> kernfs_fop_write+0x11c/0x270 ((kn->count)
>> __vfs_write+0x3c/0x70
>> vfs_write+0xcc/0x200
>> ksys_write+0x7c/0x140
>> system_call+0x5c/0x6
>>
> 
> But not the lock of the memory devices, or am I missing something?
> 

To clarify:

memory unplug will remove e.g., /sys/devices/system/memory/memoryX/,
which has a dedicated kn->count AFAIK

If you do a "echo 1 > /sys/kernel/slab/X/shrink", you would not lock the
kn->count of /sys/devices/system/memory/memoryX/, but the one of some
slab thingy.

The only scenario I could see is if remove_memory_block_devices() will
not only remove /sys/devices/system/memory/memoryX/, but also implicitly
e.g., /sys/kernel/slab/X/. If that is the case, then this is indeed not
a false positive, but something rather hard to trigger (which would
still classify as stable material).

-- 
Thanks,

David / dhildenb